To
a flight instructor, a ground instructor rating may seem about as useful
as this:

However,
the FAA wants to see that you have the ground instructor tests if you
want to become a Gold Seal flight instructor.

Additionally,
the ground instructors ratings are more stuff to put on your CV.

So,
you might as well take the ground instructor tests at the same time
as you take your flight instructor writtens. You don't have to purchase
additional software as the ground instructor tests are essentially the
same as the flight instructor tests - perhaps varying only in the number
of questions. The only ones getting slightly richer from our advice
here are the testing centers, so this is unbiased advice from us.